In a current wireless communication environment, data demand has been rapidly increased due to diversification of services and communication devices, an increase in the number of communication devices owned by individuals, and the like. In order to satisfy high data transmission and processing demand, carrier aggregation, cognitive radio technique, or the like, of effectively using various distributed frequency bands, a MIMO technique of increasing data throughput in a particular frequency band, a multi-base station cooperative transmission technique, a multi-network cooperative transmission technique, and the like, have been developed.
In general, communication standards, which are currently considered or proposed, are configured to fit a cellular structure having a form of a centralized antenna system (CAS) in which respective antenna elements are installed in groups in the same area.
In a general base station, respective antenna elements (physical antenna elements/antenna ports) are installed in groups in the same area (intervals between antenna elements are within a few wavelengths). This is called a centralized antenna system (CAS).
In comparison, when respective antenna elements are located in different areas and connected by a wired link, it is called a distributed antenna system (DAS) or a distributed multi-node system (DMNS). Here, a group of antenna elements located in the same area is called an antenna node. Namely, a CAS may be considered as a system having one antenna node, and a DMNS may be considered as a system having one or more antenna nodes.
The antenna mentioned herein may generally refer to an ‘antenna port (element) group’ or a ‘distributed antenna (DA) unit’, but it may also be substituted by an ‘antenna port’, a ‘pilot’, or a ‘reference signal (RS)’.
In a CAS, when a plurality of antenna elements exist in a single antenna node, channel ranks with respect to respective antenna elements may be changeable due to a change in small-scale channel quality. This means that the number of transmission streams (spatial streams/virtual antennas/layers) may be changed.
However, in a DMNS, one or more antenna nodes may be located to have different areal characteristics, so when the antenna nodes perform joint processing, the number of ranks may be increased due to different large scale channel quality. In this case, channel ranks of the respective antenna nodes may be changed due to the change in the small scale channel quality. Namely, in the DMNS, both large scale channel quality and small scale channel quality may affect rank adaptation.
FIG. 1 illustrates architecture of a downlink MIMO transmission end in a wireless access system.
The structure of the downlink MIMO transmission end will be described in detail with reference to FIG. 1 as follows. The transmission end includes an encoder 110, a precoder 120, subcarrier mappers 130-1, . . . , 130-K, an OFDM (Orthogonal Frequency Division Multiplexing) signal generator, Nt number of transmission antennas 500-1, . . . , 500-Nt.
The encoder 110 encodes a data stream desired to be transmitted according to a predetermined coding scheme to form coded data, modulates the coded data, and disposes the same as symbols expressing positions in signal constellation. There is no limitation in a modulation scheme, and m-PSK (m-Phase Shift Keying), m-QAM (m-Quadrature Amplitude Modulation), or the like may be used to modulate the coded data. In order to modulate the coded data, the encoder 110 may include a modulator as an independent module. Meanwhile, the encoder 110 may define an input symbol layer in order to distribute particular antenna symbols to a path of a pertinent antenna.
A layer refers to an information path of information input to the precoder 120, and an information path before the precoder 120 may be called a virtual antenna or layer. In order to define a symbol layer, the encoder 110 may include a layer mapper as an independent module.
The precoder 120 may process an input symbol according to a MIMO scheme according to the multiple transmission antennas 500-1, . . . , 500-Nt to output particular antenna symbols, and distributes the particular antenna symbols to pertinent subcarrier mappers 130-1, . . . , 130-K. Mapping of MIMO streams to antennas is performed by the precoder 120, and here, an output x of the encoder 110 is multiplied by Nt×Mt precoder w and an output from the precoder may be represented as Nt×MF matrix z.
                    x        =                  Wx          =                      [                                                                                                                                                       z                                            ⁢                      1                                        ,                    1                                                                                                                                                               z                                            ⁢                      1                                        ,                    2                                                                    —                                                                                                                                             z                                            ⁢                      1                                        ,                                          N                      F                                                                                                                                                                                                               z                                            ⁢                      2                                        ,                    1                                                                                                                                                               z                                            ⁢                      1                                        ,                    2                                                                    —                                                                                                                                             z                                            ⁢                      2                                        ,                                          N                      F                                                                                                                    —                                                  —                                                  —                                                  —                                                                                                                        N                      t                                                                                            z                                        ,                    1                                                                                                              N                      t                                                                                            z                                        ,                    2                                                                    —                                                                                            N                      t                                                                                            z                                        ,                                          N                      F                                                                                            ]                                              [                  Equation          ⁢                                          ⁢          1                ]            
Here, Nt indicates the number of transmission antennas, and zj, k indicates an output symbol to be transmitted through jth physical antenna on kth subcarrier. The precoder matrix w may be set as a matrix informed by a base station to a terminal within a predefined codebook, may be selectively determined according to a resource index in the predefined codebook, or may be set by estimating a downlink reference signal and subsequently selecting or calculating an appropriate precoder. Also, instead of defining a precoder matrix between a base station and a terminal, a terminal may randomly set a precoder matrix such that an output from the precoder is a virtual 1Tx antenna.
The subcarrier mappers 130-1, . . . , 130-K allocate the particular antenna symbols to appropriate subcarriers and multiple the same according to users. The OFDM signal generator modulates the particular antenna symbols according to OFDM and outputs OFDM symbols. The OFDM signal generator may perform IFFT (Inverse Fast Fourier Transform) on the particular antenna symbols, and a CP (Cyclic Prefix) may be inserted into a time domain symbol which has undergone the IFFT.
The OFDM symbols are transmitted through respective transmission antennas.
For reference, the transmission end may operate in two types of modes, namely, a single codeword (SCW) mode and a multi-codeword (MCW). In the SCW mode, a single codeword is transmitted through a plurality of layers created by the MIMO system regardless of the number of layers, and in the MCW mode, a single codeword is transmitted through each of a plurality of layers created by the MIMO system.
In the MCW mode, a receiver may be able to determine whether or not each codeword has been successfully decoded through a CRC (Cyclic Redundancy Check) applied to each codeword, so the reception side can obtain an additional through a reception process such as interference cancelation.
Thus, besides modules for performing demodulation, channel decoding, and multiplexing, the receiver operating in the MCW mode may further include an interference canceller for canceling interference.
In FIG. 1, in the CAS, the number of MIMO streams is determined by small-scale channel quality, and in the DMNS, the number of MIMO streams is determined by both small-scale channel quality and large-scale channel quality. The MIMO streams are mapped to actual antennas through the precoder. Namely, the number of antennas may be different from the number of transmission streams.
In the CAS having a single DA, a difference in large-scale channel quality between pertinent antenna elements is so small enough to be negligible. Thus, in the CAS, an areal channel geometry (or areal channel characteristics) or large-scale channel quality between respective antenna elements and a terminal is similar, so there is not much difference in uplink power of received by the respective reception antenna element.
Meanwhile, in the case of the DMNS, there may be wide distance variations between respective DAs and a terminal, so an areal channel geometry between the respective DAs and the terminal may be different. Such areal characteristics between the DAs and the terminal mean that the number of antenna elements of the base station is fixed with respect to the base station in the case of the existing CAS, but may vary in the case of the DMNS.
Namely, in the DMNS, the number of transmission antennas and reception antennas of the base station viewed from the terminal may be variable according to a location of the terminal and a channel state, and here, actually used antennas are called effective (physical) antenna elements.
As described above, the CAS and the DMNS have different antenna configurations and different channel characteristics due to the different antenna configurations, and thus, an existing power control algorithm and mechanism designed for the general CAS lack ability to guarantee performance in the DMNS.